Olefins, such as ethylene and propylene, are valuable hydrocarbons that are used for production of products such as polyethylene and polypropylene. Olefins are typically produced by thermal cracking of a hydrocarbon feedstock. In a thermal cracking process, heavier hydrocarbons such as naptha undergo cracking at elevated temperatures to produce olefins containing from 2 to 4 carbon atoms.
Several processes exist for cracking heavier hydrocarbons to produce olefins. In one process that is commonly used, the feedstock to be converted is heated in a furnace by passing the feedstock through the furnace within a plurality of coils. The coils are arranged to enhance heat transfer from the interior of the furnace to the feedstock within the coil. The feedstock is heated and cracked, and the cracked effluent in the outlet from the coil is quenched to terminate the cracking reaction.
The cracking of hydrocarbons in this manner results in the formation of various by-products, including coke. Coke forms on the internal surfaces of the coil and inhibits heat transfer from the furnace to the hydrocarbon feedstock. The amount of coke formed in the coils is directly related to the conversion level of the hydrocarbon feedstock. Because radiant heat is supplied to the metal coils, coke deposition inhibits heat transfer and causes the temperature of the metal coils to rise, which can result in damage to the coils. At some point, the coke fouling inhibits heat transfer to the point that the coils must be taken off-line for decoking. Decoking is typically performed using steam and air to burn the coke off of the interior of the coils. Because the decoking process requires the equipment to be taken off-line, production of olefins from the reactor halts during the decoking process.
In order to reduce the quantity of coke formed, dilution steam may be added to the feedstock. For example, in one prior process, a hydrocarbon feedstock enters a pyrolysis furnace through one or more coils in a convection section of the furnace. Dilution steam is added to each coil such that a constant steam-to-feed ratio is maintained, typically in the range of 0.3 to 0.6 pounds of steam per pound of hydrocarbon feed. The steam/feed mix may be further heated in the convection section of the furnace before entering the radiant section, where the steam/feed mix is heated to the temperature required for cracking and conversion of the hydrocarbons to olefins. The dilution steam in the mixture reduces coke formation in the tubes. The effluent from the coils is then quenched and the raw product is sent for storage or processing.
Even with the use of dilution steam, coke formation is a problem. In some processes, adiabatic reactors have been used downstream of a pyrolysis furnace to allow improved conversion of hydrocarbons to olefins, while reducing fouling of the coils in the radiant zone. In these processes, a pyrolysis furnace of the type described above is used, and the reaction conditions, in particular temperature and flow rate, are controlled to reduce the conversion of the hydrocarbon to olefins within the coils in the furnace. The reduced conversion within the coils results in reduced coke formation. A downstream adiabatic reactor is used to further convert the feedstock to olefins, thereby improving the overall conversion. Even in these processes, coke formation requires periodic down time for decoking.
In the processes described above, heavier hydrocarbons such as naptha are used as the feedstock. The use of lighter hydrocarbons such as methane or natural gas as a feedstock to produce olefins has been limited because conversion of methane requires an initiator or relatively high temperatures (greater than 1100° C.). The temperatures required are greater than those typically obtained in a pyrolysis furnace. For example, the Benson process to produce olefins from methane uses chlorine as a free radical initiator at high temperatures. This process creates very corrosive conditions, and is therefore expensive and difficult to operate.
Another process used to convert methane to olefins is oxidative coupling of methane. In this process, the methane is partially burned, and a suitable catalyst is required to promote the conversion reaction.
Because methane and natural gas are abundant and relative inexpensive compared to other hydrocarbons, it would be desirable to have an improved process for conversion of methane and natural gas to olefins. It would also be desirable to have a process for cracking naptha or other hydrocarbons that resulted in reduced down time of the reactor or pyrolysis furnace for decoking.